Many processes are carried out under high and even ultra high vacuum conditions within a chamber of a vessel representing a rigid wall structure enclosing an interior space. To observe e.g. a deposition operation in the manufacture of semiconductor devices while the operation is being carried under high vacuum conditions at high temperature a view port is built into a flange assembly of the rigid wall structure to permit spectral observation, optical thermometry and optical reflectivity. The view port is composed of a glass composition or of sapphire depending upon the thermal and gaseous environment in the chamber and the degree of vacuum. Heating the view port is known to prevent condensation build up which obscures the optical clarity of the view port. Heretofore the view port was heated by means of a wire filament of preferably tantalum attached to the view port and to the power feed through of the vacuum apparatus. The joining of a wire filament as practiced in the prior art to cause heating of the view port is difficult, expensive and unreliable.